In microwave heating, an object of microwave responsive material is placed in a cavity within the microwave field that occurs in a frequency band of 300 Megahertz (MHz) to 100 Gigahertz (GHz). The microwave energy in the field couples into the material of the object and raises the temperature. The coupling of the microwave energy is influenced by the focusing of the energy and the position within the cavity. This produces differences in coupling which in turn results in localized temperature variations in objects with different materials and different shapes.
One way of compensating for differences in shape and density in an effort to improve temperature uniformity is shown in U.S. Pat. No. 4,351,997, wherein shielding portions of an object with microwave opaque material is employed. The shielding of a portion of an object, however, produces a temperature gradient in the object which in some situations may be undesirable.
One illustration of the precision involved in the application of microwave radiation to chemical and physical processes is shown in U.S. patent application Ser. No. 07/551,716 filed Jul. 11, 1990.
A need is developing in the art for techniques that will minimize temperature differences and permit control to closer specifications in microwave processing.